One of the difficulties in blow molding plastic containers is the problem of maintaining uniform wall thickness throughout the height of the container. Even with well made preforms, slight deviations from ideal temperature distribution in the preform can cause nonuniformity and thin areas or regions in the sidewalls. Because of this problem it is usually necessary to use more resin than is needed theoretically in order to maintain minimum wall thickness specifications, resulting in higher production costs.
Currently, it is necessary for someone to pull sample containers from the output conveyor, cut them open, and manually measure the vertical wall thickness distribution. If the distribution is not satisfactory, the measurement data can indicate how the process needs to be adjusted. In some cases, there is a critical location where one measurement can indicate how the process is running. For example, if one area of the sidewall is running too thick, the additional thickness can indicate that some other area where the wall thickness is difficult to measure, such as the heel, is running too thin.
Wall thickness distribution information must be acquired as quickly as possible. If the manufacturing process is out of specification limits, a large quantity of scrap can be generated before the necessary information can be acquired and a correction made. The sectioning and manual measurement of the wall thickness distribution can be tedious, time consuming, and inaccurate. Consequently, the manufacturing process results in a large variation in container wall thickness, rarely approaching the optimum distribution necessary for achieving a desired minimum weight of the final product.
There are numerous off-line optical and mechanical systems which can be used to measure the wall thickness at various locations on a container. However, these generally require the container to be removed from the manufacturing machine, loaded upon or within the measuring system, and a large number of measurements made off-line. These systems are normally found in the quality control laboratory and cannot provide timely feedback to the forming process.
An object of the invention is to produce a plastic container wall thickness measuring system which minimizes production costs.
Another object of the invention is to produce a plastic container wall thickness measuring system, which minimizes product waste.
Still another object of the invention is to produce a plastic container wall thickness measuring system which minimizes production line downtime.
Another object of the invention is to produce a plastic container wall thickness measuring system, which can operate on-line and at a high speed during a plastic container manufacturing process.
A further object of the invention is to produce a plastic container wall thickness measuring system which can measure through both walls of the container simultaneously, and determine the average wall thickness by dividing the measurement by two.
Yet another object of the invention is to produce a plastic container wall thickness measuring system, which can make multiple thickness measurements at a given height on the container.
Still another object of the invention is to produce a plastic container wall thickness measuring system, which can make multiple thickness measurements from multiple heights on the container.